1. Field of the Invention
The invention relates to crystal growth apparatus and a method of growing crystals. More particularly, the invention relates to apparatus and method for enabling automatic control of crystal diameter during crystal growth. The invention also relates to a crucible for use in growing crystals.
2. Discussion of the Prior Art
The growth of low defect single crystals has been the subject of considerable research in, for example, the semiconductor industry. Such crystals are an essential precursor in the fabrication of a vast variety of semiconductor devices.
The Czochralski seed-pulling technique for growing single crystals is well-known [e.g. Z. Physik. Chem. (Liebzig) 92, 219 (1918)]. By this technique the seed crystal is brought into contact with molten material (melt) to facilitate further crystallisation. The crystal so produced is drawn from the melt as it grows. The double crucible method for crystal growth has also been developed [e.g. Journal of Applied Physics, 29 no. 8 (1958) pp 1241-1244 and U.S. Pat. No. 5,047,112]. The apparatus typically comprises an outer crucible containing melt of the same composition as the crystal to be grown. An inner crucible floats on the melt in the outer crucible and a small channel through the bottom of the side wall of the inner crucible allows melt to flow in from the outer crucible.
More recently a modified double crucible method has been described (BG 9412629.9) in which an injector is used to allow molten material to enter the first crucible is formed from a material of higher thermal conductivity than the material used to form the second crucible. The injector is configured to provide relatively high thermal contact with the molten material in the first crucible and relatively low thermal contact with the material in the second crucible.
An important aspect of any crystal growth technique is the visualisation, measurement and control of the crystal diameter throughout the growth process. The most frequently used method for automatic diameter control is that of weighing the crystal or the crucible [e.g. H. J. A. van Dick et al., Acta Electronica 17 1 45-55 (1974). U.S. Pat. No. 2,908,004]. By this method, the rate of change of weight is measured and is used to calculate the crystal diameter. This technique does, however, have several disadvantages.
For some materials, the density of liquid is greater than that of the solid at the melting point. Therefore, if the temperature of the melt increases and the meniscus height increases correspondingly, the crystal weight appears to increase despite the reduction in diameter which occurs as a result of the increased temperature. The effect is to provide a control signal of the opposite sign to that required. This problem occurs for most of the group III-V semiconductors, including indium antimonide. Furthermore, the crystal is subject to a downward force due to surface tension, which is proportional to the cosine of the angle of contact relative to the vertical axis of the crystal. In some materials, including most of the group III-V semiconductors, the effective angle of contact of the liquid and the crystal is positive. For an increase in temperature, the meniscus diameter just below the crystal melt interface decreases and the effective angle of contact relative to the vertical axis of the crystal is reduced. The apparent weight due to the surface tension therefore increases and therefore gives a control signal of the opposite sign to that required by the reduction of diameter. These problems are addressed in GB 1494342 and GB 1465191.
Another problem with the weighing method of automatic crystal diameter control is that the differentiation effectively amplifies any noise in the weight signal. Therefore, at slow growth rates, when the differential weight signal is small the effect of noise in the weight signal is increased and the signal to noise ratio for the differential worsens. In practice, this means the method is of little use at growth rates of less than 2 millimeters/hour. Also, for the growth of larger crystals the weighing apparatus must have a greater capacity and thus, inevitably, a lower resolution. This makes the control of growth at small diameters less precise. In the case of encapsulated melts, the encapsulant exerts a buoyancy force which effectively reduces the weight of the crystal. For example, this effect varies depending on the encapsulant depth, crystal diameter and drainage of the encapsulant off the crystal.
X-ray imaging techniques have also been used for automatic crystal diameter control [e.g. H. J. A. van Dick et al., Acta Electronica 17 1 45-55 (1974), However, this techniques also has several disadvantages. There is a radiation hazard risk associated with X-rays and radiation protection costs can be expensive. Furthermore, the technique can be inconvenient due to the size of the equipment required and the need for it to be fitted around crystal growth apparatus. X-ray transparent windows are also required. The costs of such imaging apparatus are expensive.
Optical methods for visualising crystal growth have also been employed. Known optical techniques consist principally of two methods. One technique makes use of a light beam or beams reflected off the meniscus near to the growth interface. Movement of the meniscus and change of diameter is detected by changes in the angle of the reflected beam [e.g. e.g. H. J. A. van Dick et al., Acta Electronica 17 1 45-55 (1974), U.S. Pat. No. 3,201,650]. The second method makes use of a video image of the growing crystal to detect the meniscus and determine the diameter by image processing [e.g. D. F. O""Kane et al., Journal of Crystal Growth 13/14 624-628 (1972)]. However, these methods suffer from one or more of the following disadvantages. The apparent diameter of the crystal in the image is affected by changes of depth due to the falling melt level as the crystal grows and the melt is depleted. Also, if the crystal diameter is reduced significantly suddenly, the meniscus disappears from view under the growing crystal and measurement and control are lost. In the case of a liquid encapsulated melt, reflections from the encapsulant and its meniscus can cause confusion. As the melt level falls the view of the meniscus can be obscured by the crucible wall.
Changes in the melt depth during the growth process can be compensated by means of external pulling mechanism. However, this requires additional equipment and not all effects of the falling melt depth can be overcome. For example, it is not possible to overcome obstruction of the view of the meniscus by the crucible wall. It is an object of the present invention to overcome these problems.
In Journal of Crystal Growth 13/14 619-623 (1972) Gartner et al. describe a method of viewing in which observation of the growing crystal is made at an angle of less than 15xc2x0 to the horizontal and against the bright meniscus. This method requires use of as large a crucible as possible and restricts growth to a melt fall of about 15 mm. Also, the crystal image has a dark background at the start of the growth process and a bright background later in the growth process. This background discontinuity may lead to complications and possible discontinuity of control. Also, the image of the growing crystal moves as the melt level falls and the image would move in a camera""s view unless the camera or mirror were adjusted to compensate. The mirror also tends to suffer from deterioration in reflectivity due to vapour deposition as the growth process proceeds, with volatile materials resulting in poorer control.
It is an object of the present invention to provide a crystal growth apparatus and method which overcomes these problems.
According to the present invention, crystal growth apparatus comprises:
a crucible for containing a supply of molten material from which a crystal is grown, the molten material and the crystal have a meniscus region therebetween, and
first reflection means for receiving radiation directed along an input path and reflecting radiation across a growth interface region and
second reflection means for receiving radiation reflected across the growth interface region and reflecting output radiation along an output path,
wherein the first and second reflection means are arranged at or in close proximity with the surface of the molten material such that during crystal growth they maintain a substantially constant position relative to the surface of the molten material.
The apparatus provides the advantage that an image of the crystal or any other part of the growth interface region, obtained by viewing output radiation reflected from the second reflection means, remains fixed in the field of view as the location of the first and second reflection means is independent of the fall in depth of the molten material which occurs as the crystal grows.
The apparatus may comprise support means for supporting the first and second reflection means, whereby the support means are arranged to float on the molten material such that during crystal growth the first and second reflection means maintain a substantially constant position relative to the surface of the molten material.
The support means may be integral with the first and second reflection means, or the first and second reflection means may be mounted on separate support means.
Alternatively, the support means may be a second, inner crucible containing molten material in fluid communication with the molten material in the first crucible, such that the first and second reflection means are supported on the inner crucible and the inner crucible floats on the molten material in the first crucible. This arrangement has all the advantages of a conventional double crucible apparatus, and also provides the further advantage that an image of the crystal or any other part of the growth interface region, obtained by viewing output radiation reflected from the second reflection means, remains fixed in the field of view independently of the fall in depth of the molten material as the crystal grows.
The first and second reflection means may be arranged such that input radiation reflected from the first reflection means is reflected to the second reflection means via the surface of the molten material.
The apparatus may also comprise image processing means for receiving output radiation and for forming an image of the crystal or any part of the growth interface region. The apparatus may further comprise means for heating the contents of the first crucible.
Preferably, the input and output paths make an angle of less than 5xc2x0 to the vertical and the input and output paths are in a substantially vertical direction.
The first and second reflection means may be plane mirrors. The apparatus may also comprise a source of radiation for directing radiation along the input path. The apparatus may also comprise one or more mirror for directing radiation from a source along the input path. This provides the advantage that the source may be located in a more convenient location. The apparatus may also comprise one or more mirror for reflecting radiation reflected from the second reflection means towards image processing means.
The apparatus may also comprise means for determining at least one of a crystal diameter measurement or a meniscus region diameter measurement from the observed image. The apparatus may also comprise feedback means for controlling crystal growth in response to the measured crystal diameter or the measured meniscus region diameter.
The first reflection means may be marked with a measurement scale to provide sealing for crystal diameter measurement or meniscus region diameter measurement. Alternatively, the apparatus may comprise means for reflecting a measurement scale in the first reflection means to provide sealing for crystal diameter measurement or meniscus region diameter measurement.
According to another aspect of the invention, a crucible for use in growing crystals from a molten material in which the crucible floats on molten material within an outer crucible is characterised in that it comprises first reflection means for receiving radiation and reflecting radiation across a growth interface region and second reflection means for receiving radiation reflected across the growth interface region and reflecting output radiation, wherein the first and second reflection means are arranged such that during crystal growth they are at or in close proximity with the surface of the molten material such that they maintain a substantially constant position relative to the surface of the molten material during crystal growth.
The crucible may comprise first and second reflective surfaces which may be an integral part of the crucible, e.g. polished surfaces or may be mounted on the crucible.
According to another aspect of the invention, a method of growing crystals comprises the steps of;
(i) heating a molten material from which the crystal is to be grown with heating means, the molten material and the crystal having a meniscus region therebetween,
(ii) directing radiation along an input path towards first reflection means for reflection across a growth interface region to second reflection means,
(iii) receiving radiation reflected from the first reflection means at the second reflection means and reflecting output radiation along an output path, and
(iv) arranging the first and second reflection means at or in close proximity with the surface of the molten material such that during crystal growth they maintain a substantially constant position relative to the surface of the molten material.
The method may comprise the step of supporting the first and second reflection means on support means arranged to float on the molten material such that during crystal growth the first and second reflection means maintain a substantially constant position relative to the surface of the molten material.
The method may also comprise the step of obtaining an image of the crystal or any part of the growth interface region using image processing means. The method may also comprise the further step of determining at least one of a crystal diameter measurement or a meniscus region diameter measurement from image processing means and controlling crystal growth in response to the measured crystal diameter or the measured meniscus region diameter.